The Notch gene encodes a heterodimeric transmembrane receptor whose structure is highly conserved among diverse multicellular organisms including flies, worms, mice, and humans. The Notch receptor is expressed on the surface of many cell types, and signals received through this receptor play an important role in diverse developmental and cellular processes including proliferation, differentiation, survival, and apoptosis. During normal mammalian lymphoid development, Notch signals are exquisitely regulated at multiple levels to direct the proper, stepwise maturation of T cells. Dysregulated Notch signaling, however, results in neoplastic transformation of T lymphoid progenitors, and causes pre-T acute lymphoblastic leukemia (T-ALL) in both humans and mice. Although activating mutations of the Notch receptor are relatively rare among human T-ALLs, many of these tumors derive from early stages of T cell development during which critical components of the Notch signaling pathway are expressed and functionally integrated. [unreadable] [unreadable] The goals of this proposal are to determine how dysregulation of the Notch signaling pathway leads to neoplastic transformation of T cell progenitors, and to determine if Notch signaling may play a broader role in the pathogenesis of human T-ALL than is currently appreciated. These studies will make use of novel small molecule and peptide inhibitors to modulate Notch signaling in cultured human and mouse cell lines, with validation of in vitro findings using animal models. Furthermore, hypotheses generated in the experimental setting will also be addressed using existing primary human tumor material obtained from discarded pathologic specimens. This work will yield novel insights into the multistep process of oncogenic transformation, particularly with respect to T lymphoid progenitors, and will generate findings that have potential for direct clinical application in the treatment of acute T cell leukemias. [unreadable] [unreadable] The specific aims of this proposal are: 1) to define Notch signaling events that are important for T cell transformation, and 2) to ascertain the role of Notch signaling in a broad range of human T-ALLs. [unreadable] [unreadable]